[oe] Simultaneous build of two intree packages

[Radek Dostal]

On Mon, 2012-05-07 at 08:46 -0700, Khem Raj wrote:
> > usually it crashes during "ln -sf target dest" which is executed
> > simultaneously with same parameters for both builds. There are other
> > issues as well in addition to this ln issue.
> 
> This is confusing why does this happen at all 

Because ln -sf first do symlink(1) unlink(2) and symlink(3) again. In
case link is created by another process between 2 and 3 than ln fails.

It is quite well described here:
http://rcrowley.org/2010/01/06/things-unix-can-do-atomically.html

I was also able to verify my finding by running two ln -sf in two for
loops in two different terminals and they were failing constantly.

So any idea on implementing locks/mutexes inside OE. If somebody can
point me in the right direction I should be able to implement and feed
back.

Thanks,
Radek